The traditional method of measuring and controlling a tanning ultraviolet (UV) light source is through the use of a timer. The timer is preset for a fixed amount of time and the UV light source is activated for the duration of the prescribed time interval.
This method has several shortcomings, for example, it does not account for variations in UV light intensity, such as created by variations in UV bulbs that arise from alternating current (AC) power line fluctuations, bulb wear, and lot-to-lot variations of bulbs and ballast circuits. Thus, for a given tanning session, the user may be subjected to more or less tanning radiation than is desired. It is possible that a tanning session based only upon time may exceed the FDA-prescribed maximum exposure defined as four times the minimal erythema dose (MED) for untanned Type II skin.